1. Field of the Invention
This invention relates to a control circuit of a pulse width modulation (hereinafter referred to as "PWM") inverter for driving a DC motor.
2. Description of the Prior Art
As well known, PWM inverters have been widely used for servo control of DC motors, and are suitable for speed control and torque control for DC motors. In particular, PWM inverters with wherein transistors that are used as the main switching elements of a main circuit can exercise servo control over the DC motor at a higher speed and with higher accuracy.
FIG. 1 shows the conventional PWM inverter with transistors being used as the main switching elements.
Referring to FIG. 1, an AC output from an AC power source 10 is converted into a direct current by a converter 12 and supplied to a main circuit 14 of a PWM inverter. This main circuit 14 has four large power transistors (hereinafter referred to as "PTR") 16 arranged in bridge connection. When PTR's 16, which are disposed at opposite sides, are simultaneously "ON" operated, a positive or negative driving current 100 is supplied to DC motor 18. During this "ON" condition, PTR's 16 are PWM switchingly-driven and consequently, driving current 100 is converted into a PWM current.
The speed of the DC motor 18 is detected by a speed detector 22 and driving current 100 is detected by a current detector 24. The speed detection signal 102 and the current detection signal 104 are supplied to a control circuit 20 where the PWM switchingly-driving of the PTR's 16 are effected.
This control circuit 20 is provided with a speed commanding circuit 26, a speed command signal 106 outputted from speed commanding circuit 26 which is compared with the speed detection signal 102 in a comparator 28. The deviation value from comparator 28 of the speed detection signal 102 from the speed command signal 106 is supplied to a current command restricting circuit 30 as a current command signal 108.
In order to prevent an unduly high current from being supplied to the PTR's 16, which could possibly damage the PTR's or to prevent an unduly high current from being supplied to DC motor 18 which could demagnetize the motor if DC motor 18 is of a magnetic type, a command restricting circuit 30 is utilized to restrict current command signal 108 to a preset current tolerance value.
A restricted current command signal 110 which is restricted to less than the current tolerance value by the current command restricting circuit 30 is supplied to a current control circuit 32 to which the current detection signal 104 is supplied from current detector 24. This current control circuit 32 extracts a current command deviation value of current detection signal 104 from restricted current command signal 110, and outputs a current control signal 112 which corresponds to the current command deviation value. This deviation value is supplied to a driver 34 so that the driving current 100 can be subjected to an integral control.
The driver 34 can PWM switchingly-drive the PTR's 16 simultaneously in response to the current control signal 112 to cause the main circuit 14 to output the positive or negative driving current 100 to DC motor 18.
As described above, in the inverter shown in FIG. 1, there is provided a speed control loop 114 which is a large loop and a current control loop 116 which is a small loop. Description will now be given of the operation of the above-described conventional inverter.
The speed detection signal 102 from speed detector 22 and speed command signal 106 from speed commanding circuit 26 are compared with each other in comparator 28, and, in accordance with a speed command deviation value obtained by the comparison, the speed control loop 114 (the large loop) of control circuit 20 exercises a servo control so that the speed of the DC motor 18 can coincide with speed command signal 106 from speed commanding circuit 26.
Furthermore, the current command signal (the speed deviation value) 108 obtained from comparator 28 is restricted to the current tolerance value by current command restricting circuit 30. The output of circuit 30 is restricted current command signal 110 which is supplied to the current control circuit 32. The current control circuit 32 is included in the current control loop (the small loop) 116, with the current detection signal 104 from the current detector 24 being inputted along with the output from command restricting circuit 30 to the current control circuit 32 to form a comparison. In accordance with the current command deviation value obtained by this comparison, the current control circuit 32 outputs the current control signal 112 to the driver 34. The current control signal 112 PWM switchingly-drives the PTR's 16 of the main circuit 14 to exercise an integral control over the driving current 100, whereby the driving current 100 is controlled to coincide with the restricted current command signal 110.
As described above, in the conventional control circuit 20, the DC motor 18 can be driven by the main circuit 14, and, at this time the switching frequencies of the PTR's 16 can be set at a value higher than that of a thyristor or the like, so that the servo control of the DC motor 18 can be more accurately and in higher responsiveness than in the case of using a thyristor as the main switching element.
However, in the conventional control circuit 20 as described above, an integral control is exercised over the driving current 100 in the current control circuit 32, whereby the responsiveness thereof is low. In consequence, there has been the problem that, even with the recent high improvements in the switching characteristics of the PTR's 16, hitherto there has not been effective utilization of the improved switching characteristics in servo-controlling DC motors at higher speed and with more accuracy.